The present invention relates to the distribution of a two-phase refrigerant mixture in the evaporator of a refrigeration system. More particularly, the present invention relates to improving the distribution of saturated two-phase refrigerant over and onto the tube bundle in a falling film evaporator used in a refrigeration chiller. With still more particularity, the present invention relates to the enhancement of a two-phase refrigerant distributor to address the significantly higher oil concentrations found in the two-phase refrigerant delivered into a falling film evaporator of a refrigeration chiller the compressor of which is of the screw type.
Environmental, efficiency and other similar issues and concerns have, in recent years, resulted in a need to re-think evaporator design in refrigeration chillers in view of making such evaporators more efficient from a heat exchange efficiency standpoint and in view of reducing the size of the refrigerant charge needed in such chillers. In that regard, environmental circumstances relating to ozone depletion and environmental warming have taken on significant importance in the past several years. Those issues and the ramifications thereof have driven both a need to reduce the amount and change the nature of the refrigerant used in refrigeration chillers. As a result, so-called falling film evaporators have begun to be used in refrigeration chillers.
While the use and application of evaporators of a falling film design in refrigeration chillers is beneficial, their design, manufacture and incorporation into chiller systems has proven challenging, particularly with respect to the need to uniformly distribute refrigerant across the tube bundles therein. Uniform distribution of the refrigerant delivered into such evaporators in a refrigeration chiller application is critical to the efficient operation of both the evaporator and the chiller as a whole and to reducing the size of the chiller""s refrigerant charge without compromising chiller reliability.
Achieving the uniform distribution of refrigerant is also a determining factor in the success and efficiency of the process by which oil, which migrates into the evaporator, is returned thereoutof to the chiller""s compressor. The efficiency of the process by which oil is returned from a chiller""s evaporator affects both the quantity of oil that must be available within the chiller in the first instance as well as chiller efficiency. Oil return and the ability of a refrigerant distributor in a falling film evaporator to uniformly distribute liquid refrigerant across the tube bundle in a falling film evaporator are made more difficult where the amount of oil entrained in the refrigerant is high.
Exemplary of a recent design for a falling film evaporator which employs a two-phase distribution system is the system set forth in U.S. Pat. No. 6,167,713 assigned to the assignee of the present invention and incorporated herein by reference. While that design has proven successful, it has been found that still further performance and efficiency improvements are possible, particularly where such distribution systems are employed in chiller systems where the oil circulation rate is relatively high.
In that regard, it has been found that because the vapor that issues out of the apertures on the underside of the refrigerant distributor in the falling film evaporator of the ""713 patent does so at low velocity, there is relatively little disruption of the liquid refrigerant droplets that flow through those same apertures. As a result of that circumstance, some of the liquid refrigerant that exits the distributor may cling or hang onto the bottom surface of the distributor rather than fall directly downwardly onto the underlying tube bundle.
Liquid refrigerant droplets which cling to or hang onto the bottom face of the distributor can be driven across the undersurface of the distributor by the vaporized refrigerant gas that likewise exits the apertures in the bottom face of the distributor as that gas travels along the undersurface of the distributor enroute out of the evaporator. This can lead to maldistribution of the liquid refrigerant onto the underlying tube bundle or cause it to become entrained in the refrigerant vapor which flows from the evaporator to the chiller""s compressor. If too much liquid refrigerant becomes entrained in such vapor, system efficiency and compressor reliability can be significantly affected.
Where a refrigerant distributor of the type disclosed in the ""713 patent is employed in chiller systems in which a relatively small amount of oil is entrained in the refrigerant which is delivered into the distributor, it has been found that the hang-up of liquid refrigerant on the undersurface of the refrigerant distributor can be reduced by forming a downwardly depending collar around the apertures through which refrigerant issues from the distributor undersurface. However, where distributors of the type set forth in the ""713patent are employed in chiller systems in which a relatively large amount of oil is entrained in the refrigerant that flows through the distributor, it has been found that the formation of such downwardly depending collars, of itself, may not reduce liquid refrigerant hang-up on the undersurface of the distributor to a sufficient degree.
In particular, it has been determined that the liquid refrigerant hang-up issue in chillers which employ compressors of the screw type can be significantly more severe than is the case in chiller systems of the type which employ centrifugal compressors. This is for the reason that the refrigerant gas discharged from screw compressors contains a relatively much larger amount of entrained oil then does the refrigerant gas discharged from compressors of other types, such as compressors of the centrifugal type.
The significantly higher oil concentration found in screw compressor based chiller systems results from the fact that oil is directly injected into the refrigerant undergoing compression within a screw compressor for multiple purposes, including sealing and cooling, whereas no such use of injected oil is made in centrifugal chiller systems. The increased concentration of oil found in the refrigerant that makes its way into the evaporator of a chiller system in which a compressor of the screw type is employed will therefore tend to have a higher concentration of oil in it, despite the upstream use of an efficient oil separator, and such mixture will have a greater tendency to adhere to the undersurface of the distributor in the falling film evaporator in such chiller systems.
The need therefore exists for an improved falling film evaporator for use in refrigeration chiller systems in which the refrigerant delivered into the chiller""s evaporator contains a relatively large amount of entrained oil and for an improved refrigerant distributor which better achieves the uniform distribution of liquid refrigerant to the evaporator""s tube bundle.
It is an object of the present invention to provide a falling film evaporator for use in a refrigeration chiller in which a two-phase mixture of refrigerant, in which oil is entrained, is more uniformly distributed into heat exchange contact with the evaporator""s tube bundle.
It is still another object of the present invention to reduce liquid refrigerant maldistribution across the length and width of the tube bundle of a falling film evaporator by reducing the amount of liquid refrigerant that clings to the underside of the distributor therein and is instead caused to be deposited downward and onto the underlying refrigerant tube bundle at desired locations.
A still further object of the present invention is to reduce the entrainment of liquid refrigerant in the refrigerant gas that flows out of a falling film evaporator in a refrigeration chiller system by reducing the tendency of such liquid refrigerant to hang-up on the undersurface of the refrigerant distributor employed in the evaporator.
It is still another object of the present invention to enhance the efficiency of a falling film evaporator in a refrigeration chiller of the type employing a screw compressor by causing the liquid refrigerant droplets that issue from the refrigerant distributor within the evaporator to flow from the undersurface of the distributor in a pattern that better wets the surfaces of the underlying tube bundle.
Finally, it is an object of the present invention to improve refrigerant distribution in a falling film evaporator in a chiller system in which a screw compressor is employed by providing discrete points to which liquid refrigerant is drawn and collects, as a result of surface tension forces, prior to being delivered from the refrigerant distributor to the underlying evaporator tube bundle.
These and other objects of the present invention, which will become apparent when the following Description of the Preferred Embodiment and appended drawing figures are considered, are achieved by the disposition of a refrigerant distributor in the falling film evaporator of a refrigeration chiller which, by (1) the use of staged steps of distribution internal of the distributor, (2) the maintenance of essentially constant flow velocity in the refrigerant mixture in each of the initial stages of the distribution process, (3) the arrest of the mixture""s kinetic energy in a final stage of distribution, prior to its issuance from the distributor, and (4) the direction and collection of liquid refrigerant to and at discrete points on the undersurface of the distributor results in the reduction of liquid refrigerant hang-up on the undersurface of the distributor, the expression of more uniform quantities of liquid refrigerant along the length and width of the evaporator""s tube bundle, the improved wetting of the surfaces of the tube bundle and a reduction in the entrainment of such liquid droplets within the refrigerant vapor that flows out of the evaporator to the chiller""s compressor.
Such more uniform distribution is achieved by first axially flowing the two-phase refrigerant mixture within the distributor through a passage the geometry of which maintains the flow velocity thereof essentially constant then flowing the refrigerant transversely internal of the distributor through passages of similar geometry which likewise maintains refrigerant flow therein at essentially constant velocity. The kinetic energy of the refrigerant is then absorbed, prior to its expression out of the distributor and into contact with the evaporator""s tube bundle, in what can be categorized as a third stage of distribution internal of the distributor, so that the liquid refrigerant delivered out of the apertures defined in the undersurface of the distributor is in the form of relatively large, low energy droplets. Further, by the use of downwardly depending collars around the apertures through which liquid refrigerant is delivered out of the distributor that are specially configured to make use of the surface tension forces that exist within that liquid to draw the liquid to discrete collar locations, such oil-laden liquid refrigerant is better prevented from becoming entrained in the refrigerant vapor that flows out of the evaporator, is better prevented from clinging to and being driven along the undersurface of the distributor, is better directed to intended locations on the tube bundle and better wets the upper tube surfaces thereof.